In telecommunications systems, particularly in cellular systems, the same radio frequency usually has to be divided between several users. One method of dividing the same transmission frequency between several users is code division multiple access CDMA, where different users are distinguished from one another by multiplying the signal of each user by a separate code, which differs from the other codes and is preferably orthogonal to them so that different transmissions would not correlate with one another on the radio path. In systems based on the code division multiple access, one carrier frequency is modulated by a combined signal which consists of several signals directed to different users. In the existing systems several code groups are available, such as an orthogonal variable spreading factor OVSF used as the channelization code in WCDMA systems (Wide Band Code Division Multiple Access).
The combined signal is amplified to provide it with a suitable transmission power by a power amplifier, which is linear only in a certain power range. This causes problems because the power of the combined signal may momentarily receive values that require a large linear range. Power amplifiers of this kind are difficult to design as well as expensive, for which reason the peak-to-mean ratio (peak-to-average ratio, crest factor) of the signal (or amplitudes) to be amplified has to be limited. Several limiting methods have been devised. These methods are generally called clipping methods. Usually, however, the prior art methods change a combined signal so that the orthogonality of different user-specific codes disappears. In some cases the power or amplitude of outputs cannot in practice be limited because this could hinder successful detection at the receiver of a subscriber terminal. This results from the use of a multilevel modulation method where symbols are so close to one another in the signal space diagram that even a small increase in noise causes an error in detection. In that case clipping should be directed only to the transmissions that can be clipped. Systems that cannot stand clipping include high speed downlink packet access (HSDPA) which has been researched by the 3GPP (3rd Generation Partnership Project) standardization forum. This system employs 16-level or 64-level quadrature amplitude modulation QAM.